In many industries, there is a requirement to pressure test pipes, tubes, pipe modules, pipelines and the like. For example, in the oil and gas industry, some process plant is assembled from modules, which are welded together on site. The pipe weldments in these modules must be pressure tested prior to shipping to site. The first step in the pressure testing process is the sealing of the open end of the tubing. At present, this is achieved by one of welding a cap on to the tubing, inserting an isolation plug into the end of the tubing, or by sliding a mechanical cap over the end of the tubing.
Welding a cap on to the tubing is expensive and time consuming. Furthermore, once the test is complete, the cap must be cut off.
The use of an internal isolation plug requires provision of a plug with a locking mechanism and seal. This requires a straight length of pipe to accommodate the plug, and thus may require a straight spool to be welded to the pipe to accommodate the plug. Furthermore, steel pipe tends to be supplied in standard outside diameters, but internal diameters may vary. Accordingly, internal plugs must be dressed or sized to suit the particular internal diameter of the tubing under test. This is particularly important for the plug locking mechanism, where an incorrect sizing selection may reduce the ability of the plug to resist the test pressure load, which load will tend to eject the plug from the tubing if not resisted. In the case of self-energized plugs in which the test pressure energizes the plug sealing and lock mechanisms, the loads generated by the test pressure are transferred to the tubing, causing peak hoop stress at the seal and lock contacts. Accordingly, the tube end must be assessed for each application, to ensure that the tubing wall will accommodate the anticipated hoop stress.
The use of externally mounted mechanical caps removes the dressing requirement for each tubing bore, as pipe is normally specified by outside diameter, but as the cap seals engage the outer wall surface of the tubing, and thus encompass a larger area, the ejection load experienced by a cap is greater than that experienced by a corresponding plug. Caps also tend to be heavier and require even longer lengths of straight pipe than a corresponding plug. Furthermore, the cap locks and seals create localised stresses on the tubing wall, such that the tubing wall condition must always be carefully assessed.